Well, I'm confused, then. I thought that "of the editors" was a prepositional phrase and since the subject can't be the object of a preposition, then I thought that "All" would be the subject. I believe you would diagram it that way.

I'm not sure if the "of" is implied in the second one or acting as an adjective, like you said.

It does seem that more writing teachers want you to leave off the "of" when using "all" in that way. That makes it seem like the same thing, but I guess the true question is how you would diagram it!

"All of the editors" would have a prepositional phrase diagram, while "All" and "the" of the second one would be diagrammed as adjectives.

I second SusanS's opinion. "All" has to be the subject in the first example, because the object of the preposition cannot be the subject.

However, in the second example, there is no prepositional phrase, so "editors" can be the subject, and "all" the modifier. Functionally the two sentences are the same as far as meaning goes, but not grammar.

Consider using "Some" or "None" in place of "All" and this becomes clear:

Some of the editors at the magazine agreed.

"Some" is clearly the subject.

If you substituted "some" in the second example, you get:

Some editors at the magazine agreed.

...and in this case, "editors" would be the subject, and "some" a modifier, much as if you had a number, like "Five editors..."

This is definitely the way I was taught grammar (pretty explicitly, in multiple languages), but since language is always evolving I could be behind the current standard.

I did get a perfect score on the SAT though, so I'll stick to my position until proved wrong:-)

Semantically, the sentences are identical, and "all" is a quantifier. Syntactically, "all" is the subject of the first sentence, and "of the editors" is a prepositional phrase modifier of "all". In the second sentence, "all" is a quantifier of the noun phrase "the editors". With definite plural noun phrases like "the editors," both "all" and "all of" are possible (with no semantic differences), so it is as if "of" is optional or implied. But with an indefinite noun phrase, only "all" works: